Cardiovascular and thrombus imaging agents, methods and kits

ABSTRACT

The invention provides cardiovascular imaging agents comprising a radionuclide in association with a nucleotide polyphosphate targeting molecule. Methods for using the cardiovascular imaging agents and kits containing the cardiovascular imaging agents or components suitable for production of the cardiovascular imaging agents are also provided.

This application is a 371 of PCT/US 96/20675 filed Dec. 27, 1996 and claims benefit of provisional application Ser. No. 60/009,516 filed Dec. 28, 1995.

The present invention is in the field of nuclear medicine. More specifically, the invention relates to diagnostic imaging of cardiovascular tissue and thrombi.

BACKGROUND OF THE INVENTION

Clinical imaging technology plays a significant role in diagnosis of injuries and disease processes. Many parts of the human body can now be examined for diagnostic purposes using a variety of imaging techniques. Radiography has long been used to image body parts through which externally generated x-rays are transmitted. Computerized axial tomography (CAT) provides cross-sectional x-ray images of a plane of the body. Specific tissues or organs may be targeted in positron emission tomography (PET), single photon emission computed tomography (SPECT), and gamma scintigraphy. In PET, SPECT, and gamma scintigraphy, radiopharmaceutical agents capable of being sequestered (concentrated) to some degree in the target tissue or organ are internally administered to the patient, and images are generated by detecting the radioactive emissions from the concentrated radiopharmaceutical agent. Some of the radiopharmaceutical agents currently used for cardiovascular imaging include nuclides such as ²⁰¹Tl, ^(99m)Tc, ¹³³Xe, and the like; chelates of nuclides; radiolabeled metabolic agents such as ¹¹C-deoxy-D-glucose, ¹⁸F-2-fluorodeoxy-D-glucose, [1-¹¹C]- and [¹²³I]-β-methyl fatty acid analogs, ¹³N-ammonia, and the like; infarct avid agents such as ^(99m)Tc-tetracycline, ^(99m)Tc-pyrophosphate, ²⁰³Hg-mercurials, ⁶⁷Ga-citrate, and the like; and radiolabeled ligands, proteins, peptides, and monoclonal antibodies. Whole cells such as erythrocytes, platelets, leukocytes, and other cells may also be labeled with a radionuclide and function as radiopharmaceutical agents.

The amount and type of clinical information that can be derived from PET, SPECT, and gamma scintigraphic images is related in part to the ability to concentrate the radiopharmaceutical agent in the target tissue or organ. Although many radiopharmaceuticals are available for clinical use, the resolution of the image generated may be limited depending on various factors. The resolution of a particular imaging agent for imaging diseased or injured tissue depends in part on the affinity of the radiopharmaceutical for the site of injury or disease as compared to its affinity for surrounding healthy tissue.

Radiopharmaceuticals are used in a variety of types of cardiovascular studies to obtain different kinds of information. For example, radiopharmaceutical agents used in cardiac blood flow and blood pool studies provide information on murmurs, cyanotic heart disease, and ischemic heart disease. Perfusion scintigraphy agents provide measurements of blood flow useful in detection of coronary artery disease, assessment of pathology after coronary arteriography, pre- and postoperative assessment of coronary artery disease, and detection of acute myocardial infarction. Infarct avid agents are used for “hot spot” infarct imaging. Radionuclide-containing antibodies directed against the heavy chain of cardiac myosin have been proposed to identify zones of acute myocardial necrosis, and ^(99m)Tc-labeled low density lipoprotein were proposed for detecting atheromatous lesions in their early stages after onset of endothelial damage.

Radiopharmaceutical ligands specific for β-adrenergic receptors demonstrate uptake in lungs and do not show sufficient specificity for heart tissue, as reviewed in Elmaleh, D. R., et al., in Noninvasive Imaging of Cardiac Metabolism, E. E. van der Wall, ed. (Martinus Nijhoff, Boston, 1987) pp. 1-37. The same reference describes preliminary studies of labeled muscarinic receptor ligands for cardiac imaging which showed some specificity for heart. Studies using ¹¹¹In-labeled insulin to image heart insulin receptors demonstrated less specificity for cardiovascular tissue.

Diadenosine 5′, 5′″, P¹,P⁴-tetraphosphate (Ap₄A) is an adenine analog which is ubiquitously present in living cells, appearing to play an important role in extracellular signaling events in a variety of tissues. In particular, Ap₄A is a competitive inhibitor of adenosine diphosphate (ADP)-induced platelet aggregation, which occurs through the binding ADP to a specific class of purine receptors found on platelets and megakaryocytes. U.S. Pat. No. 5,049,550 discloses antithrombotic analogs of Ap₄A, the therapeutic efficacy of which is premised on the observation that thrombus (blood clot) formation includes an initial platelet aggregation step and on the hypothesis that inhibition of platelet aggregation will result in inhibition of clot formation. U.S. Pat. No. 5,219,841 discloses a pharmaceutical composition containing Ap₄A as its active ingredient, for treatment of heart disease. U.S. Pat. No. 5,380,715 discloses use of Ap₄A as a hypotensive agent, particularly in connection with surgical procedures which employ hypotensive anesthesia.

D. R. Elmaleh, et al. (1984) Proc. Natl. Acad. Sci. USA 81, 918-921 discloses ^(99m)Tc-labeled Ap₄A (^(99m)Tc-Ap₄A) used to image tumors implanted into rats. The method used to chelate the ^(99m)Tc to the Ap₄A in this study yielded a mixture, in which ^(99m)Tc was attached to the Ap₄A-dinucleotide and which also may have contained unchelated ^(99m)Tc. This study was based on the premise that some human tumor cells are permeable to exogenous ATP and ADP, and that these cells incorporate the intact nucleotides in intracellular pools, in contrast to normal cells. Ap₄A was shown to permeate into hepatoma cells but not into a number of untransformed mammalian cell lines. In addition to accumulating in implanted tumors, the ^(99m)Tc-Ap₄A in the 1984 study also accumulated in kidney, liver, bone, muscle, and lung. No accumulation of ^(99m)Tc-Ap₄A in heart was observed in this study.

SUMMARY OF THE INVENTION

Radionuclide-labeled nucleotide polyphosphates accumulate with high specificity in atherosclerotic lesions and in heart tissue, and one embodiment of the invention generally features cardiovascular (which can include thrombi) imaging agents which include a radionuclide associated with a nucleotide polyphosphates, the latter being a targeting moiety.

In another embodiment, the invention provides a cardiovascular imaging agent that includes a radionuclide associated with a nucleotide polyphosphate targenting moiety. Typically, the targeting moiety is a residue of a targeting precursor; for example, a targeting precursor is reacted with a labeling entity which includes the radionuclide and a chelator for the radionuclide. The imaging agent is the reaction product which includes a residue of the targeting precursor and the chelator, in association with the radionuclide. The association may involve one or more of: chelation, co-valent bonding or electrostatic bonding, or it may involve other forces or combination of forces which maintains the nucleoside in spatial proximity to a targeting molecule. The imaging agent may be the reaction product of the above defined targeting precursor with a radionuclide-containing moiety, and such reaction may involve the formation of a chelate or a co-valent reaction product, or a product in which both chelation and co-valent bonds are involved. Typically, the targeting precursor is a molecule of the formula A), B), C), or D):

wherein,

(1) each of Nu₁-Nu₄ is an independently selected nucleoside;

(2) p is selected from the group consisting of a phosphate moiety, a phosphorothioate moiety, an alkylphosphonate moiety, a phosphorodithioate moiety, a phosphoramidate moiety, an aminoalkylphosphoramidate moiety, an aminoalkylphophotriester moiety, an aminoalkylphosphorothioamidate moiety, and a thiophosphate moiety;

(3) each of X, X₁, X₂, and X₃ is selected from the group consisting of an alkyl group, a halogenated alkyl group, a nitrogen-containing alkyl group, a sulfur-containing alkyl group, an alkylene group, a halogenated alkylene group, a nitrogen-containing alkylene group, and a sulfur-containing alkylene group;

(4) (n+m) is from 2 to 8; and (r+q) is from 2 to 8. Other agents (or residues of them reacted with the targeting moiety) to promote chelation or bonding may be present in the imaging agent.

In a preferred formula according to B), X is not optional, and Nu₁ and Nu₂ are the same and are adenosine, guanine, cytidine, thymidine, uracil, or inosine.

Preferably, at least one of Nu₁-N₄ (most preferably each of Nu₁-Nu₄) is adenosine. The radionuclide may be ¹²³I, ^(99m)Tc, ¹⁸F, ⁶⁸Ga, ⁶²Cu, and ¹¹¹In, although ^(99m)Tc is preferred. If the agent includes a chelating structure, particularly for ^(99m)Tc, the chelating structure may be an —N₂S₂ structure, an —NS₃ structure, an —N₄ structure, an isonitrile, a hydrazine, an NYNIC (hydrazinonicotinic acid) containing structure, a 2-methylthiolnicotinic acid containing structure, a phosphorus containing group, or a carboxylate group. The preferred moieties for X (when it is present) are alkyl moieties or chloroalkyl moieties, and p is preferably a phosphate moiety. When X is not present, the radionuclide-containing structure is chelated via an oxygen atom of the phosphate(s). Preferably, the nucleoside is adenosine, p is a phosphate, n=2, and m=2.

In one specific embodiment, the agent is a ^(99m)Tc complex having the following formula:

where Ad is adenosine, p is PO₂H, and X is a ^(99m)Tc-containing moiety.

The above agents may be administered to image cardiovascular tissue in a mammal. A specific cardiovascular imaging method detects cardiovascular lesions by administering the imaging agent to the mammal and detecting the spatial distribution of the agent accumulated in the mammal's cardiovascular system. Differential accumulation of the agent is indicative of a lesion.

The above agents may be used to image a thrombus in a mammal by administering the agent to the mammal and detecting its accumulation in a thrombus. High accumulation in a region relative other regions is indicative of the location of a thrombus.

The invention also features kits for cardiovascular imaging which comprises the imaging agent. The kit may include a chelating agent and/or an auxiliary molecule selected from the group consisting of mannitol, gluconate, glucoheptonate, and tartrate; and a tin-containing reducing agent, such as SnCl₂ or tin tartrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which:

FIG. 1 shows an HPLC chromatogram of a reaction mixture obtained when 50 mg mannitol was used as an auxiliary molecule. Peaks of the elution profile are extrapolated, with areas under the peaks of the chromatogram tabulated below the elution profile.

FIG. 2 shows an HPLC chromatogram of an alternative reaction mixture, in which 10 mg mannitol was used as an auxiliary molecule. Peaks of the elution profile are extrapolated, with areas under the peaks of the chromatogram tabulated below the elution profile.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The patent and scientific literature referred to herein establishes the knowledge that is available to those with skill in the art. The issued U.S. patents and allowed applications cited herein are hereby incorporated by reference.

The present invention provides novel cardiovascular imaging agents in which analogs of nucleotide polyphosphates are associated with a radionuclide. The imaging agents of the invention specifically accumulate in cardiovascular tissue, i.e. in heart, in arteries, and in veins. The imaging agents of the invention also accumulate in cardiovascular lesions and at sites where thrombi or emboli have become established.

The imaging agents of the invention contain a targeting molecule which specifically accumulates in cardiovascular tissue. In general, preferred targeting molecules have the formula (or dimers or other multimers of it)

Nu—(p)n—X—(p)m—Nu

wherein Nu is a nucleoside selected from the group consisting of adenosine, guanine, cytidine, thymidine, uracil, and inosine; p is selected from the group consisting of a phosphate moiety, a phosphorothioate moiety, an alkylphosphonate moiety, a phosphorodithioate moiety, a phosphoramidate moiety an aminoalkylphosphoramidate moiety, an aminoalkylphosphotriester moiety, and an aminoalkylphosphorothioamidate moiety; X is selected from the group consisting of an oxygen, an alkyl group, a halogenated alkyl group, a nitrogen-containing alkyl group, a sulfur-containing alkyl group, an alkylene group, a halogenated alkylene group, a nitrogen-containing alkylene group, and a sulfur-containing alkylene group, and (n+m) is from 2 to 8. The cardiovascular imaging agent of the invention may include formulations of more than one nuclide-labeled targeting molecule.

Any nucleoside may be used as the “Nu” component of the targeting molecule. In addition to the ribonucleosides listed above, the “Nu” component of the targeting molecule may be a deoxynucleoside, a unsubstituted ribonucleoside, or a substituted deoxyribonucleoside, a substituted ribonucleoside, or a substituted deoxyribonucleoside. Substitutions may occur in accordance with the invention on the nucleic acid base portion of the nucleoside or on the sugar portion of the nucleoside. Sugars other than ribose or deoxyribose may also be present in the “Nu” component of the targeting molecule. Preferably, the nucleoside of the targeting molecule is adenosine.

Any molecule may be used as the “p” moiety to link the nucleoside components of the targeting molecule. The targeting molecule may contain synthetic internucleoside linking moieties other than the “p” moieties listed above, in which the “p” moiety is a substituted phosphate. For example, the substituted phosphate “p” moiety may contain one of more alkyl groups, carbamate groups, acetamidate groups, and the like. The “p” moiety may additionally be a non-phosphorus containing group such as a carboxymethyl ester or a carbonate, so long as the conformation of the resulting targeting molecule approximates that of a regulatory dinucleoside such as Ap₄A.

Any moiety may be used as the “X” component of the targeting molecule, so long as the conformation of the resulting targeting molecule approximates that of a regulatory dinucleoside such as Ap₄A or dimers of Ap₄A.

Methods for synthesizing the targeting molecule are generally set forth in G. M. Blackburn, et al. (1986) in Biophosphates and Their Analogues—Synthesis, Structure, Metal and Activity, eds. Bruzik, K. S. & Stec, W. J. (Elsevier, amsterdam) pp. 451-464; G. M. Blackburn, et al. (1987) Nucleic Acids Res. 17, 6991-7004; A. Guranowski, et al. (1987) Biochemistry 26, 3425-3429; and G. M. Blackburn, et al. (1992) in Dinucleoside Polyphosphates, ed. McLennan, A. G. (CRC, Boca Raton, Fla.) Chapter 11.

In accordance with the invention, the targeting molecule is in association with (spatial proximity to) the nuclide. Spatial proximity between the targeting molecule and the nuclide may be effected in any manner which preserves the specificity of the targeting molecule for its target tissue. For example, spatial proximity between the nuclide and the targeting molecule may be effected by a covalent or non-covalent chemical bond. Such a chemical bond may be effected through a chelating substance and/or an auxiliary molecule such as mannitol, gluconate, glucoheptonate, tartrate, and the like. Alternatively, spatial proximity between the nuclide and the targeting molecule may be effected by incorporating the nuclide and the targeting molecule in a micelle or liposome, in such a way that the affinity of the targeting molecule for its target tissue is maintained. Spatial proximity between the nuclide and the targeting molecule may also be effected by attaching the nuclide and the targeting molecule to a matrix such as a microsphere or liposomes.

Those skilled in the art will recognize that there are a number of ways to synthesize the imaging agents. In one synthesis, Nu—p₂—CHCl—p₂Nu is reacted with X—R, where X is defined as X₁₋₃, above, and R is a radionuclide chelating agent, such as a ^(99m)Tc chelating agent. Typical ^(99m)Tc chelating agents include allyl or aryl amines or alkyl or aryl thiols. Other ^(99m)Tc chelating groups include N₂S₂, S₄. A precursor will be formed which contains the —R chelating agent, which in turn is reacted with ^(99m)Tc from a standard ^(99m)Tc reduction kit. For example, when —R is an alkyl thiol, the —NS₃ chelating reaction forms:

If the composition includes a chelating structure, particularly for ^(99m)Tc, the chelating structure may be an N₂S₂ structure, an —NS₃ structure, an N₄ structure, an isonitrile, a hydrazine, a NYNIC (hydrazinonicotinic acid) group, a phosphorus containing group, or a carboxylate group. Those skilled in the art will understand that a great many other chelating reactions may be used.

The imaging agents described above may contain any radionuclide in accordance with the invention. Preferably, the imaging agents of the invention contain radionuclides suitable for use in PET or SPECT imaging. More preferably, the imaging agent of the invention contains a radionuclide selected from the group consisting of ¹²³I, ^(99m)Tc, ¹⁸F, ⁶⁸Ga, ⁶²Cu, ¹¹¹In, and the like. Such radionuclides may be incorporated into the imaging agent by covalent bonding directly to an atom of the targeting molecule, or the radionuclide may be non-covalently or covalently associated with the targeting molecule through a chelating structure or through an auxiliary molecule such as mannitol, gluconate, glucoheptonate, tartrate, and the like. When a chelating structure is used to provide spatial proximity between the radionuclide and the targeting molecule, the chelating structure may be directly associated with the targeting molecule or it may be associated with the targeting molecule through an auxiliary molecule such as mannitol, gluconate, glucoheptonate, tartrate, and the like.

Any suitable chelating structure may be used to provide spatial proximity between the radionuclide and the targeting molecule of the agent through covalent or non-covalent association. Many such chelating structures are known in the art. Preferably, the chelating structure is an N₂S₂ structure, an NS₃ structure, an N₄ structure, an isonitrile-containing structure, a hydrazine containing structure, a HYNIC (hydrazinonicotinic acid) group-containing structure, a 2-methylthiolnicotinic acid group-containing structure, a carboxylate group containing structure, and the like. In some cases, chelation can be achieved without including a separate chelating structure, because the radionuclide chelates directly to atom(s) in the targeting moiety, for example to oxygen atoms in the phosphate group(s) or in carboxylate group(s).

The chelating structure, auxiliary molecule, or radionuclide may be placed in spatial proximity to any position of the targeting molecule which does not interfere with the interaction of the targeting molecule with its receptor in cardiovascular tissue. Accordingly, the chelating structure, auxiliary molecule, or radionuclide may be covalently or non-covalently associated with any moiety of the targeting molecule except the receptor-binding moiety. For example, the chelating structure, auxiliary molecule, or radionuclide may be associated with the phosphate moiety of the targeting molecule, with the —X— moiety of the targeting molecule.

Radionuclides may be placed in spatial proximity to the targeting molecule using known procedures which effect or optimize chelation, association, or attachment of the specific radionuclide to ligands. For example, when ¹²³I is the radionuclide, the imaging agent may be labeled in accordance with the known radioiodination procedures such as direct radioiodination with chloramine T, radioiodination exchange for a halogen or an organometallic group, and the like. When the radionuclide is ^(99m)Tc, the imaging agent may be labeled using any method suitable for attaching ^(99m)Tc to a ligand molecule. Preferably, when the radionuclide is ^(99m)Tc, an auxiliary molecule such as mannitol, gluconate, glucoheptonate, or tartrate is included in the labeling reaction mixture, with or without a chelating structure. More preferably, ^(99m)Tc is placed in spatial proximity to the targeting molecule by reducing ^(99m)TcO₄ with tin in the presence of mannitol and the targeting molecule. When Ap₄A or an analog of Ap₄A is the targeting molecule, the cardiovascular imaging agent of the invention is preferably made by reducing about fifty to 100 mCi ^(99m)TcO₄ with 0.05-1 mg SnCl₂ in the presence of about 1 to 20 mg of mannitol, for each mg of Ap₄A or Ap₄A analog. More preferably, for each mg of Ap₄A or Ap₄A analog, about 0.05 mg SnCl₂ and about 10 mg mannitol are used to reduce the ^(99m)TcO₄. Other reducing agents, e.g., tin tartrate, may also be used to make the cardiovascular imaging agent of the invention. An exemplary procedure for placing ^(99m)Tc in spatial proximity to Ap₄A and the Ap₄A analog in which the “X” moiety is —CHCl— is set forth in Example 1.

After the labeling reaction is complete, the reaction mixture may optionally be purified using one or more high performance liquid chromatography (HPLC) steps. Any suitable HPLC system may be used if a purification step is performed, and the yield of cardiovascular imaging agent obtained from the HPLC step may be optimized by varying the parameters of the HPLC system, as is known in the art. Any HPLC parameter may be varied to optimize the yield of the cardiovascular imaging agent of the invention. For example, the pH may be varied, e.g., raised, to decrease the elution time of the peak corresponding to the cardiovascular imaging agent of the invention.

The invention as embodied in a kit for imaging comprises one or more of the imaging agents described above, in combination with a pharmaceutically acceptable carrier such as human serum albumin. Human serum albumin for use in the kit of the invention may be made in any way, for example, through purification of the protein from human serum or though recombinant expression of a vector containing a gene encoding human serum albumin. Other substances may also be used as carriers in accordance with this embodiment of the invention, for example, detergents, dilute alcohols, carbohydrates, auxiliary molecules, and the like. The kit of the invention may of course also contain such other items as may facilitate its use, such as syringes, instructions, reaction vials, and the like.

In one embodiment, a kit according to the invention contains from about 1 to about 30 mCi of the radionuclide-labeled cardiovascular imaging agent described above, in combination with a pharmaceutically acceptable carrier. The kits of the invention may include radionuclide-labeled Ap₄A analogs. The cardiovascular imaging agent and carrier may be provided in solution or in lyophilized form. When the cardiovascular imaging agent and carrier of the kit are in lyophilized form, the kit may optionally contain a sterile and physiologically acceptable reconstitution medium such as water, saline, buffered saline, and the like.

In another embodiment, the kit of the invention may contain the unlabeled targeting molecule which has been covalently or non-covalently combined with a chelating agent; an auxiliary molecule such as mannitol, gluconate, glucoheptonate, tartrate, and the like; and a reducing agent such as SnCl₂ or tin tartrate. In this embodiment, the kit may include unlabeled Ap₄A or an unlabeled Ap₄A analog. The unlabeled targeting molecule/chelating agent and the auxiliary molecule may be present as separate components of the kit or they may be combined into one kit component. The unlabeled targeting molecule/chelating agent, the auxiliary molecule, and the reducing agent may be provided in solution or in lyophilized form, and these components of the kit of the invention may optionally contain stabilizers such as NaCl, silicate, phosphate buffers, ascorbic acid, gentisic acid, and the like. Additional stabilization of kit components may be provided in this embodiment, for example, by providing the reducing agent in an oxidation-resistant form. Determination and optimization of such stabilizers and stabilization methods are well within the level of skill in the art. When the unlabeled targeting molecule/chelating agent of this embodiment are in lyophilized form, the kit may optionally contain a sterile and physiologically acceptable reconstitution medium such as water, saline, buffered saline, and the like. The amounts of unlabeled targeting molecule/chelating agent, auxiliary molecule, and reducing agent in this embodiment are optimized in accordance with the methods for making the cardiovascular imaging agent set forth above. Radionuclides, such as ^(99m)Tc from a commercially available ⁹⁹Mo/^(99m)Tc generator or commercially available ¹²³I, may be combined with the unlabeled targeting molecule/chelating agent and the reducing agent for a time and at a temperature sufficient to chelate the radionuclide to the targeting molecule/chelating agent, and the imaging agent thus formed is injected into the patient.

The cardiovascular imaging agents of the invention may be used in accordance with the methods of the invention by those of skill in the art, e.g., by specialists in nuclear medicine, to image cardiovascular tissue in a mammal. Any mammalian cardiovascular tissue may be imaged the imaging agents of the invention. For example, the cardiovascular imaging agents of the invention are suitable for imaging the heart, vascular tissues surrounding the heart, vascular tissues of the lung, vascular tissues of the extremities, cerebrovascular tissues, and the like. In accordance with the invention, the cardiovascular imaging agents described herein may be used to detect cardiovascular lesions such as thrombi, embolisms, infarcts, areas of ischemia, and the like, in a mammal. The methods of the invention may employ radionuclide-labeled Ap₄A or a radionuclide-labeled Ap₄A analog.

Images are generated by virtue of differences in the spatial distribution of the imaging agents which accumulate in the various tissues and organs of the mammal. The spatial distribution of the cardiovascular imaging agent accumulated in a mammal, in an organ, or in a tissue may be measured using any suitable means, for example, a gamma camera, a PET apparatus, a SPECT apparatus, and the like. Some cardiovascular lesions may be evident when a less intense spot appears within the image, for example, within a ^(99m)Tc-Ap₄A-labeled heart, indicating the presence of tissue in which a lower concentration of imaging agent accumulates relative to the concentration of imaging agent which accumulates in surrounding cardiovascular tissue. Alternatively, a cardiovascular lesion might be detectable as a more intense spot within the image, indicating a region of enhanced concentration of the imaging agent at the site of the lesion relative to the concentration of agent which accumulates in surrounding cardiovascular tissue. Thrombi and embolisms are examples of cardiovascular lesions which accumulate enhanced concentrations of the imaging agents of the invention. Accumulation of lower of higher amounts of the imaging at the site of a lesion may readily be detected visually, by inspection of the image of the cardiovascular tissue. Alternatively, the extent of accumulation of the imaging agent may be quantified using known methods for quantitating radioactive emissions. A particularly useful imaging approach employs more than one imaging agent to perform simultaneous studies. For example, simultaneous studies of perfusion and metabolic function would allow study of coupling and uncoupling of flow of metabolism, thus facilitating determinations of tissue viability after a cardiac injury. Such determinations are useful in diagnosis of cardiac ischemia, cardiomyopathy, tissue viability, hybrinating heart, and other heart abnormalities.

The cardiovascular imaging agents of the invention are used in the following manner. An effective amount of an imaging agent comprising at least one targeting molecule and a nuclide (from 1 to 50 mCi) may be combined with a pharmaceutically acceptable carrier for use in imaging studies. In accordance with the invention, “an effective amount” of the imaging agent of the invention is defined as an amount sufficient to yield an acceptable image using equipment which is available for clinical use. An effective amount of the imaging agent of the invention may be administered in more than one injection. Effective amounts of the imaging agent of the invention will vary according to factors such as the degree of susceptibility of the individual, the age, sex, and weight of the individual, idiosyncratic responses of the individual, the dosimetry. Effective amounts of the imaging agent of the invention will also vary according to instrument and film-related factors. Optimization of such factors is well within the level of skill in the art.

As used herein, “pharmaceutically acceptable carrier” includes any and all solvents, dispersion media, coatings, antibacterial and antifungal agents, isotonic agents, absorption delaying agents, and the like. The formulation used in the present invention may also contain stabilizers, preservatives, buffers, antioxidants, or other additives known to those of skill in the art. The use of such media and agents for pharmaceutically active substances is well known in the art. Supplementary active compounds can also be incorporated into the imaging agent of the invention. The imaging agent of the invention may further be administered to an individual in an appropriate diluent or adjuvant, co-administered with enzyme inhibitors or in an appropriate carrier such as human serum albumin or liposomes. Pharmaceutically acceptable diluents include sterile saline and other aqueous buffer solutions. Adjuvants contemplated herein include resorcinols, non-ionic surfactants such as polyoxyethylene oleyl ether and n-hexadecyl polyethylene ether. Enzyme inhibitors include pancreatic trypsin inhibitor, diethylpyrocarbonate, and trasylol. Liposomes inhibitors include water-in-oil-in-water CGF emulsions as well as conventional liposomes (Strejan et al. (1984) J. Neuroimmunol. 7, 27).

Preferably, the imaging agent of the invention is administered intravenously, and the imaging agent will be formulated as a sterile, pyrogen-free, parenterally acceptable aqueous solution. The preparation of such parenterally acceptable solutions, having due regard to pH, isotonicity, stability, and the like, is within the skill in the art. A preferred formulation for intravenous injection should contain, in addition to the cardiovascular imaging agent, an isotonic vehicle such as Sodium Chloride Injection, Ringer's Injection, Dextrose Injection, Dextrose and Sodium Chloride Injection, Lactated Ringer's Injection, or other vehicle as known in the art.

The amount of imaging agent used for diagnostic purposes and the duration of the imaging study will depend upon the nature and severity of the condition being treated, on the nature of therapeutic treatments which the patient has undergone, and on the idiosyncratic responses of the patient. Ultimately, the attending physician will decide the amount of imaging agent to administer to each individual patient and the duration of the imaging study.

The following examples are included for illustrative purposes only and are not intended to limit the scope of the invention.

EXAMPLE 1 Synthesis of ^(99m)Tc-Ap₄A-coligand (Mannitol or Gluconate) and ^(99m)Tc-Ap₂CHClp₂A-coligand (Mannitol or Gluconate)

Fifty to 100 mCi ^(99m)TcO₄ was obtained from a ⁹⁹Mo/^(99m)Tc generator (DuPont Merck, Billerica, Mass.) in 1.5 ml sterile water and mixed with a solution of one mg Ap₄A (Sigma, St. Louis, Mo.), 50 mg mannitol, and 100 μg SnCl₂ in 200 μl sterile water. The solution was stirred and allowed to react for ten minutes at room temperature. The reaction mixture was then HPLC purified by adsorption onto a 5 mm diameter, 25 cm long C8-ODS reverse phase column (Waters, Milford, Mass.) and elution with CH₃CN: buffer (20:80). The buffer for the elution step contained 3.1 ml concentrated H₃PO₄, 3.9 ml t-butylammonium hydroxide, which were combined and titrated with NaOH to pH 2.4. FIG. 1 shows an elution profile using this procedure, with unlabeled material shown by the bold line(measured by A₂₅₄), and labeled material shown in the fainter solid line (measured by τ-emissions). The dotted line in FIG. 1 is the radioactivity elution profile. The radioactive peak eluting at 15.826 minutes is ^(99m)Tc-Ap₄A, which was injected into rabbits as set forth in Example 2, to image cardiovascular tissue. The radioactive peak eluting at 1-3 minutes on this chromatogram was ^(99m)Tc-mannitol, which was used as a control in the experiments described in Example 2.

A similar procedure was used to label the Ap₄A analog of the formula:

A—(p)₂—CHCl—(p)₂—A

with ^(99m)Tc.

FIG. 2 is a reverse phase HPLC elution profile of a ^(99m)Tc-Ap₄A preparation in which 10 mg mannitol and 40 μg SnCl₂ was used to perform the reduction of one mg Ap₄A. This reaction mixture was also stirred and allowed to react for ten minutes at room temperature. HPLC purification of the reaction mixture was performed as above, with the exception that the CH₃CN:buffer ratio was 15:85 and the elution buffer contained 3.0 ml concentrated H₃PO₄ and 3.36 ml t-butylammonium hydroxide titrated with NaOH to pH 3.4. FIG. 2 shows the elution profile of the reaction mixture, the peak eluting at 13.430 minutes corresponding to ^(99m)TcAp₄A.

When one mg mannitol was used as an auxiliary molecule in the labeling reaction, very low yields ^(99m)Tc-Ap₄A were obtained.

EXAMPLE 2 Imaging of Experimental Atherosclerotic Lesions in Rabbits

Experimental atherosclerotic lesions were induced in rabbits by deenodotheliazation of the infradiaphragmatic aorta followed by a 6% peanut oil-2% cholesterol diet as set forth in Narula, et al., Circulation 92, 474-484 (1995). Two or four mCi of the ^(99m)Tc-Ap₄A, the ^(99m)Tc-labeled Ap₄A analog produced in Example 1, or ^(99m)Tc-mannitol was injected into the marginal ear veins of rabbits in which experimental lesions had been induced and into control animals which had not been subjected to the experimental lesion procedure. Serial gamma images were collected every minute for the first five minutes after injection, every two minutes for the next 30 minutes, and every five minutes for the next three hours, in the anterior and lateral decubitus views using a standard field-of-view gamma camera (Series 100, Ohio Nuclear) equipped with a medium-energy, parallel-hole collimator. The pulse height analyzers were adjusted to record the ^(99m)Tc photopeak. Planar images of the heart and the damaged aorta were obtained in a 256×256 matrix. The rabbits were then sacrificed, and the heart, aorta, and other tissues were removed, washed with saline, weighed, and counted in an automatic well-type gamma counter (model 1282, LKB Compugamma, Helsinki, Finland). The ^(99m)Tc-Ap₄A and ^(99m)Tc-labeled Ap₄A analog rapidly accumulated in atherosclerotic lesions, showing retention in the lesions for more than three hours. The ratio of radioactivity in the lesions to radioactivity in blood in one hour was more than six to one. The ^(99m)Tc-Ap₄A and 99mTc-labeled Ap₄A analog also accumulated in heart tissue, but not in other tissues. In contrast, no radioactivity accumulated in the lesions or in heart tissue when ^(99m)Tc-mannitol was injected. Control animals showed no accumulation of ^(99m)Tc-Ap₄A or ^(99m)Tc-labeled Ap₄A analog in these tissues.

Although a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. For example, many other chemical groups are interchangeable on the targeting molecule with the various substituted moieties without significantly altering the activity of the targeting molecule for diagnostic imaging purposes. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. 

What is claimed is:
 1. A cardiovascular imaging agent comprising a radionuclide associated with a targeting moiety by way of an auxiliary molecule, said targeting moiety comprising a residue of nucleotide polyphosphate precursor.
 2. A cardiovascular imaging agent comprising a radionuclide associated with a targeting moiety by way of an auxiliary molecule, said targeting moiety comprising a residue of nucleotide polyphosphate precursor having formula A), B), C), or D):

wherein, in the above formulas A)-D), (1) each of Nu₁-Nu₄ is selected from the group consisting of adenosine, guanosine, cytidine, thymidine, uridine, and inosine; (2) p is selected from the group consisting of a phosphate moiety, a phosphorathioate moiety, an alkylphosphonate moiety, a phosphorodithioate moiety, a phosphoramidate moiety, an aminoalkylphosphoramidate moiety, an aminoalkylphosphotriester moiety, an aminoalkylphosphorothioamidate moiety, and a thiophosphate moiety; (3) each of X, X₁, X₂, and X₃ is selected from the group consisting of an alkylene group, and a halogenated alkylene group; and (4) (n+m) is from 2 to 8; and (r+q) is from 2 to
 8. 3. The agent of claim 2 in which each of Nu₁-Nu₄ is independently selected from the group nucleosides consisting of adenosine, guanosine, cytidine, thymidine, uridine, and inosine.
 4. The agent of claim 2 wherein at least one of Nu₁-N₄ is adenosine.
 5. The agent of claim 4 in which each of Nu₁-Nu₄ is adenosine.
 6. The agent of claim 1 or claim 2, further including a radionuclide selected from the group consisting of ¹²³I, ^(99m)Tc, ¹⁸F, ⁶⁸Ga, ⁶²Cu, and ¹¹¹In.
 7. The agent of claim 2, wherein said imaging agent comprises the product of combining said polyphosphate precursor with a chelating structure which chelates said radionuclide.
 8. The agent of claim 7, wherein the chelating structure is selected from the group consisting of an —N₂S₂ structure, an —NS₃ structure, an —N₄ structure, an isonitrile, a hydrazine, a HYNIC group-containing structure, 2-methylthiolnicotinic acid-group, and a carboxylate group-containing structure.
 9. The agent of any of claims 7-8, wherein the radionuclide is ^(99m)Tc.
 10. A cardiovascular imaging agent comprising a radionuclide associated with a targeting moiety, said targeting moiety comprising a residue of nucleotide polyphosphate precursor having formula A), B), C), or D):

wherein, in the above formulas A)-D), (1) each of Nu₁-Nu₄ is an independent nucleoside; (2) p is a phosphate moiety; (3) X is an alkyl moiety or a chloroalkyl moiety; and (4) (n+m) is from 2 to 8; and (r+q) is from 2 to
 8. 11. A cardiovascular imaging agent comprising a complex of ^(99m)Tc having the formula:

where Ad is adenosine, and p is PO₂H, and X is a ^(99m)Tc-containing chelating moiety.
 12. A method of imaging cardiovascular tissue in a mammal which comprises administering to the mammal a cardiovascular imaging agent comprising a radionuclide associated with a targeting moiety, said targeting moiety comprising a residue of nucleotide polyphosphate precursor, and obtaining an image of the cardiovascular tissue.
 13. The method of claim 12 in which the method detects a cardiovascular lesion in a mammal, said method comprising the steps of administering to the mammal the imaging agent, detecting the spatial distribution of the agent accumulated in the mammal's cardiovascular system, wherein a detected accumulation of a agent in a region which is different from the detected accumulation of agent in other regions in indicative of a lesion.
 14. A method of imaging a thrombus in a mammal which comprises administering to the mammal an a cardiovascular imaging agent comprising a radionuclide associated with a targeting moiety, said targeting moiety comprising a residue of nucleotide polyphosphate precursor, and obtaining an image of the cardiovascular tissue.
 15. A method of imaging a cardiovascular tissue in a mammal which comprises administering to the mammal a cardiovascular imaging agent comprising a radionuclide associated with a targeting moiety, said targeting moiety comprising a residue of nucleotide polyphosphate precursor having formula A), B), C), or D);

wherein, in the above formulas A)-D), (1) each of Nu₁-Nu₄ is an independently selected nucleoside; (2) p is a phosphate moiety; (3) X is an alkyl moiety or a chloroalkyl moiety; and (4) (n+m) is from 2 to 8; and (r+q) is from 2 to
 8. 16. A method of imaging a thrombus in a mammal which comprises (a) administering to the mammal a cardiovascular imaging agent comprising a radionuclide associated with a targeting moiety said targeting moiety comprising a residue of nucleotide polyphosphate precursor having formula A), B), C), or D):

 wherein, in the above formulas A)-D), (1) each of Nu₁-Nu₄ is selected from the group consisting of adenosine, guanosine, cytidine, thymidine, uracil, and inosine; (2) p is selected from the group consisting of a phosphate moiety, a phosphorathioate moiety, an alkylphosphonate moiety, a phosphorodithioate moiety, a phosphoramidate moiety, an aminoalkylphosphoramidate moiety, an aminoalkylphosphotriester moiety, an aminoalkylphosphorothioamidate moiety, and a thiophosphate moiety; (3) each of X, X₁, X₂, and X₃ is selected from the group consisting of an alkylene group, and a halogenated alkylene group; and (4) (n+m) is from 2 to 8; and (r+q) is from 2 to 8; and (b) and obtaining an image of the cardiovascular tissue.
 17. A kit for cardiovascular imaging which comprises a supply of a targeting precursor, said targeting precursor having formula A), B), C), or D):

wherein, in the above formulas A)-D), (1) each of Nu₁-Nu₄ is selected from the group consisting of adenosine, guanosine, cytidine, thymidine, uridine, and inosine; (2) p is selected from the group consisting of a phosphate moiety, a phosphorathioate moiety, an alkylphosphonate moiety, a phosphorodithioate moiety, a phosphoramidate moiety, an aminoalkylphosphoramidate moiety, an aminoalkylphosphotriester moiety, an aminoalkylphosphorothioamidate moiety, and a thiophosphate moiety; (3) each of X, X₁, X₂, and X₃ is selected from the group consisting of an alkylene group, and a halogenated alkylene group; and (4) (n+m) is from 2 to 8; and (r+q) is from 2 to
 8. 18. The kit of claim 17 wherein at least one of Nu₁-Nu₄ is adenosine.
 19. The kit of claim 17 wherein at least one of Nu₁-N₄ is adenosine.
 20. The kit of claim 17 comprising supply of a radionuclide selected from the group consisting of ¹²³I, ^(99m)Tc, ¹⁸F, ⁶⁸Ga, ⁶²Cu, and ¹¹¹In.
 21. The kit of claim 20, wherein said kit further comprises at least one chelate forming substance.
 22. The kit of claim 21, wherein the chelating forming substance is selected from the group consisting of an —N₂S₂ structure, an —NS₃ structure, an —N₄ structure, an isonitrile, a hydrazine, a HYNIC group-containing structure, 2-methylthiolnicotinic acid-group, and a carboxylate group-containing structure.
 23. The kit of any of claims 21 or 22 wherein the radionuclide is ^(99m)Tc.
 24. The kit of claim 17 in which p is a phosphate moiety.
 25. The kit of claim 17 wherein the nucleoside is adenosine, p is a phosphate, n=2, and m=2.
 26. The kit of claim 17 further comprising a chelating agent, the chelating agent comprising an auxiliary molecule selected from the group consisting of mannitol, gluconate, glucoheptonate, and tartrate, and a tin-containing reducing agent.
 27. The agent of any of claims 2-5 in which p is a phosphate moiety.
 28. The agent of claim 10 in which each of Nu₁-Nu₄ is independently selected from the group nucleosides consisting of adenosine, guanosine, cytidine, thymidine, uridine, and inosine.
 29. The agent of claim 10 wherein at least one of Nu₁-Nu₄ is adenosine.
 30. The agent of claim 29 in which each of Nu₁-Nu₄ is adenosine.
 31. A method of imaging cardiovascular tissue in a mammal which comprises (a) administering to the mammal a cardiovascular imaging agent comprising a radionuclide associated with a targeting moiety said targeting moiety comprising a residue of nucleotide polyphosphate precursor having formula A), B), C), or D):

 wherein, in the above formulas A)-D), (1) each of Nu₁-Nu₄ is selected from the group consisting of adenosine, guanosine, cytidine, thymidine, uracil, and inosine; (2) p is selected from the group consisting of a phosphate moiety, a phosphorathioate moiety, an alkylphosphonate moiety, a phosphorodithioate moiety, a phosphoramidate moiety, an aminoalkylphosphoramidate moiety, an aminoalkylphosphotriester moiety, an aminoalkylphosphorothioamidate moiety, and a thiophosphate moiety; (3) each of X, X₁, X₂, and X₃ is selected from the group consisting of an alkylene group, and a halogenated alkylene group; and (4) (n+m) is from 2 to 8; and (r+q) is from 2 to 8; and (b) and obtaining an image of the cardiovascular tissue.
 32. The method of claim 31 in which the method detects a cardiovascular lesion in a mammal, said method comprising the steps of administering to the mammal the imaging agent, detecting the spatial distribution of the agent accumulated in the mammal's cardiovascular system, wherein a detected accumulation of an agent in a region which is different from the detected accumulation of agent in other regions is indicative of a lesion.
 33. A method of imaging cardiovascular tissue in a mammal which comprises administering to the mammal a cardiovascular imaging agent comprising a complex of ^(99m)Tc having the formula:

where Ad is adenosine, and p is PO₂H, and X is a ^(99m)Tc-containing chelating moiety; and obtaining an image of the cardiovascular tissue.
 34. The method of claim 33 in which the method detects a cardiovascular lesion in a mammal, said method comprising the steps of administering to the mammal the imaging agent, detecting the spatial distribution of the agent accumulated in the mammal's cardiovascular system, wherein a detected accumulation of an agent in a region which is different from the detected accumulation of agent in other regions is indicative of a lesion.
 35. A method of imaging a thrombus in a mammal which comprises administering to the mammal a cardiovascular imaging agent comprising a complex of ^(99m)Tc having the formula:

where Ad is adenosine, and p is PO₂H, and X is a ^(99m)Tc-containing chelating moiety; and obtaining an image of the cardiovascular tissue.
 36. A method of claim 14, 16, or 35 further comprising reacting the polyphosphate precursor with a radionuclide containing moiety.
 37. The agent of claim 15 in which each of Nu₁-Nu₄ is independently selected from the group nucleosides consisting of adenosine, guanosine, cytidine, thymidine, uridine, and inosine.
 38. The agent of claim 15 wherein at least one of Nu₁-Nu₄ is adenosine.
 39. The agent of claim 38 in which each of Nu₁-Nu₄ is adenosine. 